1. Field of the Invention
The present invention is generally directed to the field of electrochemistry, and it more specifically relates to a new separation method and apparatus for removing ions, contaminants and impurities from water and other aqueous process streams, and for placing the removed ions back into solution during regeneration of the apparatus.
2. Background Art
The separation of ions and impurities from electrolytes has heretofore been generally achieved using a variety of conventional processes including: ion exchange, reverse osmosis, electrodialysis, electrodeposition, or filtering. Several other methods have been proposed and address the problems associated with the conventional separation processes. However, these proposed methods have not been completely satisfactory for specific applications nor useful for all applications, and have not met with universal commercial success or complete acceptance. One such proposed ion separation method, which relates to a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes, will be described later in more detail.
The conventional ion exchange process has been used as a means for removing anions and cations, including heavy metals and radioisotopes, from process and waste water in various industries. This process generates large volumes of corrosive secondary wastes that must be treated for disposal through regeneration processes. Existing regeneration processes are typically carried out following the saturation of the columns by ions, by pumping regeneration solutions, such as concentrated acids, bases, or salt solutions through the columns. These routine maintenance measures produce significant secondary wastes, as well as periodic interruptions of the deionization process.
Secondary wastes resulting from the regeneration of the ion exchangers typically include used anion and cation exchange resins, as well as contaminated acids, bases and/or salt solutions. For example, H.sub.2 SO.sub.4 solutions have been used for the regeneration of cation columns in metal finishing and power industries; and HNO.sub.3 solutions have been used for the regeneration of cation columns used in processing nuclear materials.
In some instances, the secondary radioactive wastes are extremely hazardous and can cause serious environmental concerns. For instance, during plutonium processing, resins and solutions of HNO.sub.3 become contaminated with PuO.sub.2.sup.++ and other radioisotopes. In this case, every pound of cation exchange resin requires approximately 100 pounds of 10 wt. % HNO.sub.3 and 2 to 3 pounds of rinse water for regeneration. Similarly, every pound of anion exchange resin requires approximately 100 pounds of 10 wt. % NaOH and 2 to 3 pounds of rinse water for regeneration. Given the high and increasing cost of disposal of secondary wastes in mined geological repositories, there is tremendous and still unfulfilled need for reducing, and in certain applications, eliminating the volume of secondary wastes.
Another example is the use of the ion exchange process for industrial purposes, such as in the electroplating and metal finishing industries. A typical electroplating process includes immersing the article to be electroplated in an electroplating bath which contains dissolved metals, such as nickel, cadmium zinc, copper, silver and/or gold, as well as a variety of salts, and then rinsing this article. Once the electroplating process is completed, the plated article is rinsed to remove residual plating solution and associated contaminants. The rinsing process includes hanging the article on a rack above a rinse tank and spraying it with rinse water from spray nozzles around the top of the rinse tank.
The rinse water becomes contaminated, and a major dilemma currently facing the industry relates to the difficulties, cost considerations and the environmental consequences for disposing of the contaminated rinse solution. A typical treatment method for the contaminated rinse water is the ion exchange process.
Other exemplary processes which further illustrate the problems associated with ion exchange include residential water softening and the treatment of boiler water for nuclear and fossil-fueled power plants. At the present time, domestic water softeners use a concentration solution of sodium chloride to regenerate a bed of ion exchange resin. Unfortunately, such water softeners result in a relatively high concentrated solution of sodium chloride in the drinking water produced by the system. Therefore, additional desalination devices, such as reverse osmosis filters are needed to remove the excess sodium chloride introduced during regeneration. It should be noted that people on low-salt diets also require low-salt water. A solution that contains contaminants from the ion exchange resin is produced during regeneration and must be discharged to the sewer.
Boiler water for nuclear and fossil-fueled power plants is treated with ion exchange to remove ionic contaminants such as Mg.sup.++, Ca.sup.++, Cu.sup.++, and Cl.sup.-, and is essential for the prevention of pitting, stress corrosion cracking, and scaling of heat transfer surfaces. Such treatment is particularly important on nuclear powered ships and submarines. Another important example is the production of high-purity water for semiconductor processing. Other applications could include the removal of toxic ions, especially those containing selenium, from waters produced by agricultural irrigation.
Therefore, there is still a significant and growing need for a new method and apparatus for deionization and subsequent regeneration, which significantly reduce, if not entirely eliminate secondary wastes in certain applications. This method and apparatus should not require salt additions for ion regeneration in a water softening system, and further should not require additional desalination devices, such as reverse osmosis filters, to remove the excess sodium chloride introduced during regeneration.
Additionally, the new method and apparatus should enable the separation of any inorganic or organic ion or dipole from any ionically conducting solvent, which could be water, an organic solvent, or an inorganic solvent. For example, it should be possible to use such a process to purify organic solvents, such as propylene carbonate, for use in lithium batteries and other energy storage devices. Furthermore, it should be possible to use such a process to remove organic ions, such as formate or acetate from aqueous streams.
The new method and apparatus should further be adaptable for use in various applications, including without limitation, treatment of boiler water in nuclear and fossil power plants, production of high-purity water for semiconductor processing, removal of toxic and hazardous ions from water for agricultural irrigation, and desalination of sea water.
In the conventional reverse osmosis systems, water is forced through a membrane, which acts as filter for separating the ions and impurities from electrolytes. Reverse osmosis systems require significant energy to move the water through the membrane. The flux of water through the membrane results in a considerable pressure drop across the membrane. This pressure drop is responsible for most of the energy consumption by the process. The membrane will also degrade with time, requiring the system to be shut down for costly and troublesome maintenance.
Therefore, there is a need for a new method and apparatus for deionization and ion regeneration, which substitute for the reverse osmosis systems, which do not result in a considerable pressure drop, which do not require significant energy expenditure, or interruption of service for replacing the membrane(s).
The conventional ion separation method relating to a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes is described in the Office of Saline Water Research and Development Progress Report No. 516, March 1970, U.S. Department of the Interior PB 200 056, entitled The Electrosorb Process for Desalting Water, by Allan M. Johnson et al., hereinafter referred to as the "Department of the Interior Report" and further in an article entitled "Desalting by Means of Porous Carbon Electrodes" by J. Newman et al., in J. Electrochem. Soc.: Electrochemical Technology, March 1971, Pages 510-517, hereinafter referred to as the "Newman Article", both of which are incorporated herein by reference. A comparable process is also described in NTIS research and development progress report No. OSW-PR-188, by Danny D. Caudle et al., Electrochemical Demineralization of Water with Carbon Electrodes, May, 1966.
The Department of the Interior Report and the Newman Article review the results of an investigation of electrosorption phenomena for desalting with activated carbon electrodes, and discuss the theory of potential modulated ion sorption in terms of a capacitance model. This model desalination system 10 is diagrammatically illustrated in FIG. 1, and includes a stack of alternating anodes and cathodes which are further shown in FIG. 2, and which are formed from beds of carbon powder or particles in contact with electrically conducting screens (or sieves). Each cell 12 includes a plurality of anode screens 14 interleaved with a plurality of cathode screens 16, such that each anode screen 14 is separated from the adjacent cathode screen 16 by a first and second beds 18, 20, respectively of pretreated carbon powder. These two carbon powder beds 18 and 20 are separated by a separator 21, and form the anode and cathode of the cell 12. In operation, and as shown in FIG. 1, raw water is flown along the axial direction of the cells 12, perpendicularly to the surface of the electrode screens 14, 16, to be separated by the system 10 into waste 23 and product 25.
However, this model system 10 suffers from several disadvantages, among which are the following:
1. The carbon powder beds 18 and 20 are used as electrodes and are not "immobilized". The primary carbon particles and fines, smaller particles generated by erosion of the primary particles, can become readily entrained in the flow, which eventually depletes the carbon bed, reduces the efficiency of the deionization or desalination system 10, and necessitates maintenance. PA1 2. It is significant that raw water must flow axially through these electrode screens 14 and 16, beds of carbon powder 18 and 20, and separators 21, which cause significant pressure drop and large energy consumption. PA1 3. The carbon bed electrodes 18 and 20 are quite thick, and a large potential drop is developed across them, which translates into lower removal efficiency and higher energy consumption during operation. PA1 4. Even though the carbon particles "touch", i.e., adjacent particles are in contact with each other, they are not intimately and entirely electrically connected. Therefore, a substantial electrical resistance is developed, and significantly contributes to the process inefficiency. Energy is wasted and the electrode surface area is not utilized effectively. PA1 5. The carbon beds 18 and 20 have a relatively low specific surface area. PA1 6. The carbon powder bed electrodes 18 and 20 degrade rapidly with cycling, thus requiring continuous maintenance and skilled supervision. PA1 7. The model system 10 is designed for one particular application, namely sea water desalination, and does not seem to be adaptable for use in other applications. PA1 1. Unlike the conventional osmosis process where water is forced through a membrane by pressure gradient, and unlike the conventional ion exchange process and the process described in the Newman Article and the Department of Interior Report, where fluid is flown through a packed bed, the present separation methods and systems do not require the electrolyte to flow through any porous media such as membranes or packed beds. In the present system, electrolyte flows in open channels formed between two adjacent, planar electrodes, which are geometrically parallel. Consequently, the pressure drop is much lower than conventional processes. The fluid flow can be gravity fed through these open channels, or alternatively, a pump can be used. PA1 2. The present system does not require membranes, which are both troublesome and expensive, which rupture if overpressured, which add to the internal resistance of the capacitive cell, and which further reduce the system energy efficiency. This feature represents a significant advantage over the conventional reverse osmosis systems which include water permeable cellulose acetate membranes, and over the electrodialysis systems which require expensive and exotic ion exchange membranes. PA1 3. The electrodes in the present system are composed of immobilized sorption media, such as monolithic carbon aerogel which is not subject to entrainment in the flowing fluid stream. Thus, material degradation due to entrainment and erosion is considerably less than in conventional packed carbon columns. This feature represents yet another significant advantage over the systems described in the Department of the Interior Report and the Newman Article where activated carbon is lost from the bed over the conventional ion exchange systems where ion exchange resin is lost from the beds. PA1 4. The present systems and methods are inherently and greatly energy efficient. For instance, in both evaporation and reverse osmosis processes, water is removed from salt, while in the present systems, salt is removed from water, thus expending less energy. PA1 5. The present systems and methods present superior potential distribution in the thin sheets of carbon aerogel. Unlike the deep, packed carbon beds used in the electrosorb process discussed in the Department of Interior Report and in the Newman Article, most of the carbon aerogel is maintained at a potential where electrosorption is very efficient. In deep, packed carbon beds, the potential drops to levels where the electrosorption process is not very effective. Furthermore, the specific surface area of the sorption media used in the present process is significantly greater than that of carbon powder. This feature also contributes significantly to overall process efficiency. PA1 1. Removal of various ions from waste water without the generation of acid, base, or other similar secondary wastes. This application may be especially important in cases involving radionuclides, where the inventive capacitive deionization process could be used to remove low-level radioactive inorganic materials. PA1 2. Treatment of boiler water in nuclear and fossil power plants. Such treatment is essential for the prevention of pitting, stress corrosion cracking, and scaling of heat transfer surfaces. Such a process may be particularly attractive for nuclear powered ships and submarines where electrical power is readily available and where there are space limitations, thereby restricting the inventory of chemicals required for regeneration of ion exchange resins. PA1 3. Production of high-purity water for semiconductor processing. In addition to removing conductivity without the addition of other chemical impurities, the system is capable of removing small suspended solids by electrophoresis. Furthermore, the organic impurities chemisorb to the carbon. PA1 4. Electrically-driven water softener for homes. The present system would soften home drinking water without the introduction of sodium chloride. At the present time, domestic water softeners use sodium chloride to regenerate a bed of ion exchange resin. Downstream of the ion exchanger, reverse osmosis has to be used to remove the sodium chloride introduced during regeneration. People on low-salt diets require low-salt water. The present capacitive deionization system does not require salt additions for regeneration, does not have to be followed by a reverse osmosis system, and will also remove hazardous organic contaminants and heavy metals from the water. PA1 5. Removal of salt from water for agricultural irrigation. The energy efficiency of such a process and the lack of troublesome membranes could make such a process a contender for treating water for irrigation purposes. Solar energy could be used to power the low-voltage, low-current capacitive deionization plants. PA1 6. Desalination of sea water. Such an application can be accomplished using the present separation method and system.
Therefore, there is still a significant unfulfilled need for a new method and apparatus for deionization and regeneration, which, in addition to the ability to significantly reduce if not to completely eliminate secondary wastes associated with the regeneration of ion exchange columns, do not result in a considerable pressure drop of the flowing process stream, and do not require significant energy expenditure. Furthermore, each electrode used in this apparatus should be made of a structurally stable, porous, monolithic solid. Such monolithic electrodes should not become readily entrained in, or depleted by the stream of fluid to be processed, and should not degrade rapidly with cycling. These electrodes should have a very high specific surface area; they should be relatively thin, require minimal operation energy, and have a high removal efficiency. The new method and apparatus should be highly efficient, and should be adaptable for use in a variety of applications, including, but not limited to sea water desalination.